an introduction to geometric design of roads and streets

An Approved Continuing Education Provider

PDHonline Course C814 (2 PDH)

An Introduction to

Geometric Design of Roads and Streets

J. Paul Guyer, P.E., R.A.

2015

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©2015 J. Paul Guyer of 41

An Introduction to Geometric Design of Roads and Streets

J. Paul Guyer, P.E., R.A.

CONTENTS

1. GENERAL

2. DESIGN CONSIDERATIONS

3. GEOMETRIC DESIGN FOR ROADS AND STREETS

(This publication is adapted from the Unified Facilities Criteria of the United States government which are in the public domain, have been authorized for unlimited distribution, and are not copyrighted.)

(Figures, tables and formulas in this publication may at times be a bit difficult to read, but they are the best available. DO NOT PURCHASE THIS PUBLICATION IF THIS LIMITATION IS UNACCEPTABLE TO YOU. )





1. 1. GENERAL

1.1 PURPOSE AND SCOPE. This publication discusses the geometric design criteria

for roads and streets. It sets forth the approaches and traffic flow criteria for guidance

in determining types and configurations best suited for construction.

1.2 DEFINITIONS OF PERTINENT TERMS. The definitions of terms relative to

highway design are given in lists of definitions presented in the manuals of AASHTO

and as a part of specific procedures described below.

1.2.1 ACCESS HIGHWAYS. An access highway is an existing or proposed public

highway which is needed to provide highway transportation services from a public or

private installation to suitable transportation facilities.

1.2.2 INSTALLATION HIGHWAYS. Installation highways include all roads and streets

within the site limits of a public or private installation.

1.2.3 HIGHWAY PLANNING. The planning of the general road system is an integral

part of public or private installation master planning. Major objectives of master

planning are the grouping of related functions reasonably close to each other and the

interrelating of land-use areas for maximum efficiency, speed of construction, and

economy of operation. The connecting road system should be planned in keeping with

these objectives to minimize travel and permit the optimum circulation of traffic

originating both outside and within the installation. Traffic studies may not be available,

so good engineering judgment and assessment of current and/or near future needs

must be made to determine traffic requirements. The geometric design of highway

facilities should provide for safe, smooth, and convenient traffic movement consistent

with time limitations, topographical conditions, and to the extent possible, economical

construction. Existing roads and streets at installations can be classif ied in accordance

with requirements presented in tables 1-1 and 1-2. The elements to be given primary

consideration in such classifications are pavement width, shoulder width, degree and





length of slopes (grade), and passing sight distance. Values for these elements should

be essentially equal to or greater than the minimum requirements for classification

assigned. All of the requirements in table 1-1 or 1-2 should be considered, but

requirements other than those just listed can be given greater latitude.

1.4 TRAFFIC. The volume and composition of the traffic determines the geometric

requirements for roads and streets. Type, volume, character, frequency, and

composition of traffic at installations are related to size, type, and purpose of the

installation. The size, type, and purpose of the installation provide information as to its.

functional requirements, indicating character and size of vehicles. Types of vehicles,

types of terrain, and frequency of use establish the traffic classification in which roads

and streets fall. Classification reflecting character of traffic is based upon the

characteristics and dimensions of existing vehicles. It is essential that the designer be

aware of the vehicular traffic anticipated prior to selection of the type design to use on

a particular project.





Table 1-1

Geometric Criteria for Classified Roads Within "Open" Areas (Rural Areas)





Table 1-1 (continued)

Geometric Criteria for Classified Roads Within "Open" Areas (Rural Areas)






Geometric Criteria for Classified Roads within "Open" Areas (Rural Areas)





Table 1-2

Geometric Criteria for Classified Street Within "Built-up" Areas






Geometric Criteria for Classified Street Within "Built-up" Areas





2. DESIGN CONSIDERATIONS

2.1 GENERAL. Geometric design deals with the dimensions of the visible features of a

facility such as alinement, sight distances, widths, slopes, and grades. Geometric

design criteria are set forth in tables 1-1 and 1-2 and discussed in subsequent

paragraphs.

2.2 ROAD AND STREET TYPES.

2.2.1 DESIGNATIONS OF TYPES. Highways are generally typed according to the

number of traffic lanes as single-, two-, and three-lane, and undivided or divided

multilane (four or more traffic lanes) highways. When information is available relative

to volume and composition of traffic and type of terrain for a proposed highway, the

type required can be readily determined by comparing the traffic volume expected on

the proposed road or street with the design hourly volume shown in tables 1-1 and 1-2.

2.2.2 SINGLE-LANE ROADS. Geometric design criteria for single-lane roads are

shown in table 1-1 under "Class E Roads -mountainous." Where shoulders are not

sufficiently stable to permit all-weather use and the distance between intersections is

greater than 1/2 mile, turnouts will be provided at 1/4-mile intervals for use by

occasional passing or meeting vehicles. Single-lane pavements may be provided for

fire lanes and approach drives to buildings within built-up areas, in which case the

pavement will be at least 12 feet wide in all cases.

2.2.3 TWO-LANE ROADS AND STREETS.

2.2.3.1 CLASS B, D, AND E ROADS. The bulk of the roads and streets at many

installations are two-lane highways. These include Class B, D, and E roads and Class

B, D, and E streets.





2.2.3.2 CLASS A, C, AND F ROADS. Road classifications A, C, and F may not be

used for some conditions. Class B roads will allow adequate traffic pattern

considerations to provide criteria for road design thicknesses commensurate with the

life expectancy of the program. The use of four lane roads is to be minimized for

mobilization construction. Where four lane roads cannot be avoided, Class B criteria

will be used. When the road classifications were reviewed in light of requirements, it

was determined that the requirements of Class B roads or Class D roads could be

used to satisfy the traffic range of Class C roads. The single lane roads of Class F can

readily be designed as minimum Class E roads. By reducing the number of road

classifications, the refinement and detail of traffic flow data is greatly reduced without

seriously affecting the development of some road systems.

2.2.4 MULTILANE (FOUR TRAFFIC LANES OR MORE) HIGHWAYS •. The design

criteria presented herein for highways are generally applicable to multilane highways

also, except that passing sight distance is not required. Where multilane highways are

designed for relatively high speeds, opposing traffic should be separated by properly

designed medians.

2.3 DESIGN CONTROLS.

2.3.1 TOPOGRAPHY AND LAND USE. Tables 1-1 and 1-2 set forth appropriate

design standards for roads and streets traversing flat, rolling, or mountainous terrain in

built-up or open areas.

2.3.2 VEHICLE CHARACTERISTICS. Table 2-1 shows dimensions of design vehicles

on which the geometric design criteria presented herein are based. Some of these

vehicles are wider than 8.5 feet, which is the maximum width shown in table 2-1 for

any of the design vehicles.

2.3.3 TRAFFIC. The geometric design criteria presented in tables 1-1 and 1-2 have

been developed on the basis of horizontal area requirements for various combinations





of number and kind of vehicles expected in the traffic stream. The general unit for

measurement of traffic is average daily traffic (ADT); the basic fundamental unit of

measurement of traffic is design hourly volume (DHV).

2.3.3.1 VOLUME. Traffic volumes are expressed as ADT and DHV in tables 1-1 and 1-

2. The ADT represents the total traffic volume for the year divided by 365. It is a value

needed to determine total service and economic justification for highways but is

inadequate for geometric design because it does not indicate the significant variation

in the traffic during seasons, days, or hours. If a road or street is to be designed so

that traffic will be properly served, consideration must be given to the rush-hour

periods. The rush hour volume represented as an average daily peak hour is the basis

of the DHV. The DHV is to be used for geometric design. Limited studies made of

traffic flows at some installations indicate that because of the high frequency with

which peak hourly traffic occurs, the average daily peak can be economically and

efficiently used as the DHV. The DHV in tables 1-1 and 1-2 are shown as 15 and 12

percent, respectively, of the ADT. These are median values selected for Army

installations.

2.3.3.2 COMPOSITION. Traffic on installation roads and streets may consist of a

combination of passenger cars, light-delivery trucks, single-unit trucks, truck

combinations, buses, and other vehicles. Trucks, buses, and other vehicles have more

severe operating characteristics, occupy more roadway space, and consequently

impose a greater traffic load on highways than do passenger cars and light-delivery

trucks. ADT and DHV for various combinations of vehicular traffic are shown in tables

1-1 and 1-2. The larger the proportions of buses, trucks, and some other vehicles

present in the traffic stream during the selected design hour, the.greater the traffic load

and highway capacity required. The DHV of tables 1-1 and 1-2 diminish for each

highway class as the percentage of buses, trucks, and tracked vehicles in the traffic

stream increase. The tables provide design data for traffic containing 0, 10, 20, and 30

percent buses, trucks, and some other vehicles.





Table 2-1

Design Vehicle Dimensions





2.4 SPEED AND CAPACITY INFLUENCE.

2.4.1 SPEED.

2.4.1.1 INFLUENCE ON GEOMETRIC DESIGN. Vehicular speed varies according to

the physical characteristics of the vehicle and highway, weather conditioning, volume

of traffic, and the type of shoulders and other roadside features. On streets, the speed

generally will depend on traffic control devices when weather and traffic conditions are

favorable. On roads, the physical features of the roadway usually control speed if other

conditions are favorable. Therefore, speed is a positive control for geometric design.

Consideration must be given to the selected design speed and average running speed

if adequate designs are to be developed.

2.4.1.2 DESIGN SPEED. The speed selected for design is the major control in

designing physical features of highways. Practically all features of a highway will be

affected to some extent by the design speed. Maximum curvature, superelevation, and

minimum sight distance are automatically determined by the selected design speed.

Other features such as pavement and shoulder width, and lateral clearance to

obstructions are not directly affected by design speed but do affect vehicle speed. The

design speed should be selected primarily on the basis of terrain characteristics, land

use, and economic considerations. The geometric design criteria presented herein are

based on the design speeds shown under "Design Controls" in tables 1-1 and 1-2.

2.4.1.3 AVERAGE RUNNING SPEED. The average running speeds on which the

geometric design criteria are based are shown under "Design Controls" in tables 1-1

and 1-2.

2.4.2 CAPACITY.

2.4.2.1 CONDITIONS AFFECTING CAPACITY. The capacity of a road or street will

vary with lane width, distance to lateral obstructions, condition and width of shoulders,





profile and alinement, and with the composition and speed of traffic. These factors are

referred to collectively as prevailing conditions. Those factors depending on physical

features of the highway are called prevailing roadway conditions, and those depending

on the character of the using traffic are called prevailing traffic conditions. The term

capacity in itself has no significance unless the prevailing roadway and traffic

conditions are stated.

2.4.2.2 CAPACITY ANALYSIS. Capacities under ideal conditions are presented in the

TRB Highway Capacity Manual. Uninterrupted flow capacities under ideal traffic and

roadway conditions for 2-lane, 2-way, highway, (total for both lanes) and for multilane

highway (average per lane for direction of heavier flow), will be 2,000 passenger cars

per hour.

2.4.2.3 CAPACITY FOR UNINTERRUPTED FLOW. The DHV shown in tables 1-1 and

1-2 are equal.to the capacity for uninterrupted flow for each class of road and street on

the basis of the geometric design criteria presented. Highway capacity is directly

related to the average running speed. Maximum capacity occurs when average

running speed is between 30 and 45 mph. Any factors which reduce or increase the

average running speed will also reduce capacity. The capacities (DHV) shown in

tables 1-1 and 1-2 for Class B roads, and Class B and D streets will be reduced in

accordance with the following tabulation in all cases where it is anticipated that the

average running speed on a substantial length of a road or street will be appreciably

less than 30 mph.





3. GEOMETRIC DESIGN FOR ROADS AND STREETS

3.1 CROSS-SECTION ELEMENTS.

3.1.1 PAVEMENT.

3.1.1.1 TYPE SURFACE. Pavement type is seldom an important factor in geometric

design; however, the ability of a pavement surface to retain its shape and dimensions,

its cross-section, and the possible effect of pavement surface on driver behavior

should be considered in geometric design.

3.1.1.2 NORMAL CROSS SLOPE. Selection of proper cross slope depends upon

speed-curvature relations, vehicle characteristics, curb requirements, and general

weather conditions. Cross slope for sharp curves (superelevation) is discussed in

AASHTO GD-2. Cross slope on tangents and flat curves are shown in tables 1-1 and

1-2. Where two or more lanes are inclined in the same direction on Class B roads and

streets, each successive lane outward from the crown line should have an increased

cross slope. The lane adjacent to the crown line should have the minimum cross slope

shown in tables 1-1 and 1-2 and the cross slope of each successive lane should be

increased 1/16 in/ft. Where pavements are designed with barrier curbs, it is

recommended that a minimum cross slope of 3/16 in/ft be used on Class B roads and

streets and that a minimum cross slope of 1/4 in/ft be used on Class D and E roads

and streets.





3.1.2 LANE WIDTH. The number and width of traffic lanes shown in tables 1-1 and 1-2

are the minimum considered adequate to accommodate the indicated design hourly

volume when the traffic is composed principally of wheeled vehicles whose overall

widths are 8.5 feet or less. Wider traffic lanes are required when the traffic is

composed of a significant percentage of vehicles whose overall widths are greater

than 8.5 feet. In general, the lane width will be increased by the excess width of the

largest over-sized vehicle (vehicle width minus 8.5 feet to the nearest higher even foot)

where such traffic is anticipated.

3.2 CURBS, COMBINATION CURBS, AND GUTTERS. Curbs, combination curbs and

gutters, and paved gutters with attendant storm drainage facilities will not be

considered during a mobilization situation unless they are determined to be absolutely

necessary or conditions would allow their construction. The road or street design will

account for the "no curb" feature and provide for drainage and erosion control

accordingly.

3.2.1 CURB CONSTRUCTION. In built-up areas, curbs, combination curbs and

gutters, and paved gutters with attendant underground storm drainage systems will be

provided when necessary along streets and in open storage areas as required to aid in

the collection and disposal of surface runoff including snowmelt, to control erosion, to

confine traffic, or as required in the extension of existing similar facilities. In open

areas, combination curbs and gutters will not be provided along roads except where

necessary on steep grades to control drainage and prevent erosion of shoulders and

fill slopes. Where such facilities are required, they should be located outside the edges

of traffic lanes and should be either of the mountable type with suitable outlets and

attendant drainpipes or paved gutters with shallow channels extending across the road

shoulders and down the fill slopes.

3.2.2 CLASSIFICATION AND TYPES. Curbs are classified as barrier or mountable

according to their intended use. Barrier curbs are designed to prevent or at least





discourage vehicles from running off the pavement and therefore have a steeply

sloping face at least 6 inches high. Mountable curbs are designed to allow a vehicle to

pass over the curb without damage to the vehicle, and have a flat sloping face 3 to 4

inches high. For construction purposes, curbs are usually designated as "combined

curb and gutter" and "integral curb and gutter." For some installations curbs are

divided into four types for convenience of reference: type I is a combined gutter

section and barrier curb; Type II is a combined gutter section and mountable curb;

Type III is a combined gutter section and offset barrier curb; and Type IV is a barrier

curb integral with pavement slab. Curbs should be placed on undisturbed subgrade or

subgrade compacted to the same density as subgrade of adjacent road.

3.2.3 LOCATION IN REGARD TO LANE WIDTH.

3.2.3.1 TYPE I, III, OR IV (BARRIER CURBS). It is necessary to offset barrier curbs a

sufficient distance from the edge of the nearest traffic lane to prevent reduction in

capacity. Curb offset and traffic lane width for classified roads and streets designed

with

barrier curbs are shown in tables 1-1 and 1-2.

3.2.3.2 TYPE II CURBS. Mountable curbs cause very little, if any, lateral displacement

of traffic adjacent to these curbs; therefore, it is acceptable to locate Type II curbs at

the edge of a traffic or parking lane.

3.3 ROAD AND STREET APPENDAGES.

3.3.1 SHOULDERS.

3.3.1.1 WIDTH. Shoulder widths should be as stated in tables 1-1 and 1-2.





3.3.1.2 SHOULDERS FOR ROADS. Roads in rural areas are normally designed

without curbs and require full width shoulders to accommodate high traffic volumes.

Geometric design criteria for shoulders on roads are presented in table 1-1.

3.3.1.3 SHOULDERS FOR STREETS. As a general rule, streets in cities are designed

with some type of barrier curb and do not ·require shoulders except where needed for

lateral support of the pavement and curb structure. Where lateral support is required,

the shoulder should be at least 4 feet in width where feasible. In other sections within

built-up areas, where desirable to design streets without barrier curbs, geometric

design criteria are presented in table 1-2.

3.3.2 MEDIANS.

3.3.2.1 USES. Where traffic volume requires construction of multilane highways,

opposing traffic should be separated by medians. Medians should be highly visible

both day and night, and there should be a definite color contrast between median and

traffic lane paving. The absolute minimum width for a median is 4 feet with a desirable

minimum width of 14 feet.

3.3.2.2 TYPES. Cross sections of medians are illustrated in figure 3-1. It is not

necessary that medians be of uniform width throughout the length of divided highways.

3.3.2.3 CURBS. All design criteria relative to curbs presented herein are applicable to

median curbs.

3.3.3. GUARDRAILS AND GUIDEPOSTS.

3.3.3.1 USES. For safety and guidance of traffic, guideposts should be provided at all

locations along roadways where drivers may become confused, particularly at night,

as to the direction of the roadway; along roadways subject to periodic flooding; along

roadways where fog exists for long periods of time; and where driving off the roadway





is prohibited for reasons other than safety. Guardrails are normally required at

locations where vehicles accidentally leaving the roadway might be damaged,

resulting in injury to occupants. Guardrails or guideposts should conform to local

highway department criteria.





FIGURE 3-1

CROSS SECTIONS OF GENERAL TYPES OF MEDIANS





FIGURE 3-1 (CONTINUED)

CROSS SECTIONS OF GENERAL TYPES OF MEDIANS





FIGURE 3-2

DESIGN CRITERIA: GUARD RAILS, GUIDE POSTS AND SIDE SLOPES





3.3.3.2 DESIGN CRITERIA. Guardrails or guideposts are not normally required where

the front side slopes are 4:1 or flatter. Design criteria for determining where guardrai ls

or guideposts are required is shown by figure 3-2. The ordinate of this figure,

designated "Height of Cut or Fill in Feet," is used in this manual to refer to the vertical

distance between the outside (intersection of shoulder and front slope planes) edge of

the shoulder and the toe of the front slope in cuts and on fills, or between the toe and

top of back slope in cuts.

3.3.3.3 LOCATION WITH RESPECT TO EDGE OF PAVEMENT. Guardrails or

guideposts should be located at a constant offset from the edge of a pavement outside

the limits of the usable shoulder and with an elevation of the base between 4 inches

and 9 inches below the edge of the lane pavement. Shoulder widths shown in tables 1-

1 and 1-2 will be widened 2 feet to provide space for installation of guardrails or

guideposts. Guardrails or alinement of guideposts should be flared outward and, if

required, buried on the traffic approach end and tapered in at narrow structures to

meet curb lines.

3.3.3.4 MARKING. Guardrails and guideposts must be highly visible, particularly at

night. All guardrails and guideposts should be marked or painted in accordance with

AASHTO safety requirements.

3.3.4 EARTH SLOPES. In determining inclination of side slopes, proper consideration

should be given to drainage, maintenance, erosion, and stability. It may be difficult to

control erosion of some soils on steep slopes (4:1 or steeper), and it may be

impossible to control erosion or maintain vegetation cover on slopes steeper than 2:1.

If maintenance is to be accomplished without special equipment, side slopes should

not be steeper than 3:1. It is essential that side slopes along highways be flat enough

to assure stability under all normal conditions. Design criteria for selecting earth slopes

is presented in combination with design policy for establishing need for guardrails and

guideposts in figure 3-2.





3.3.5 BRIDGE CLEARANCE. Requirements affecting highway safety are found in

AASHTO HB-12.

3.3.5.1 HORIZONTAL. The minimum horizontal distance between curbs on bridges

should be equal to the width of the approaching roadway including traffic lanes,

parking lanes, full width of shoulders, and medians (on divided highways). When the

cost of parapets and railings is less than the cost of decking the median area, traffic

lanes for traffic in opposing directions will be on separate structures. It is usually more

economical to pave over the median area on .bridges with a median width less than

about 15 feet.

3.3.5.2 VERTICAL. The minimum vertical clearance will be at least 14 feet over all

traffic lanes, parking lanes, and shoulders.

3.4 SIGHT DISTANCE. The length of roadway visible ahead of a vehicle along a

highway is termed "sight distance." Sight distance is divided into two categories:

stopping sight distance and passing sight distance. Minimum stopping and passing

sight distances are shown in tables 1-1 and 1-2. Effort should be made to provide sight

distances greater than those shown.

3.4.1. STOPPING SIGHT DISTANCE. On single-lane roads, the stopping sight

distance must be adequate to permit approaching vehicles each to stop. Horizontal

curve sight distance will be critical and will be twice that required for a two or more,

lane highway.

3.4.2 PASSING SIGHT DISTANCE. Passing sight distance should be provided as

frequently as possible along two-lane, two-way roads, and a length equal to or greater

than the minimum values shown in table 1-1 should be provided. The minimum

passing sight distances in table 1-1 provide safe distances for a single isolated vehicle

traveling at design speed to pass a vehicle going 10 mph less than design speed.





Sight distances and safe passing sections should be shown on all construction and

improvement plans to aid in proper marking and sign placement.

3.5 HORIZONTAL ALINEMENT.

3.5.1 GENERAL. Where changes in horizontal alinement are necessary, horizontal

curves should be used to affect gradual change between tangents. In all cases;

consideration should be given to the use of the flattest curvature practicable under

existing conditions. Adequate design of horizontal curves depends upon establishment

of the proper relations between design speed and maximum degree of curvature (or

minimum radius) and their relation to superelevation. The maximum degree of

curvature is a limiting value for a given design speed and varies with the rate of

superelevation and side friction factors.

3.5.2 MAXIMUM CURVATURE.

3.5.2.1 ROADS. Desirable and absolute values for use in design of horizontal curves

on superelevated roads are shown in table 1-1. The absolute maximum curvature for

roads without superelevation is the same as shown for streets with normal crown

sections in table 1-2.

3.5.2.2 STREETS. Absolute maximum values for degree of curvature on streets in

built-up areas are shown in table 1-2. Absolute maximum values are given for streets

with normal crown sections (no superelevation) and with superelevated sections.

3.5.3 SUPERELEVATION. A practical superelevation rate together with a safe side

friction factor determines maximum curvature. Superelevation rate and side friction

factors depend upon speed, frequency and amount of precipitation and type of area,

i.e., built-up or open. Superelevation rates will be determined in accordance with

AASHTO GD-2.





3.5.4 WIDENING OF ROADS AND STREETS.

3.5.4.1 PAVEMENTS ON ROADS AND STREETS will be widened to provide

operating conditions on curves comparable to those on tangents. Widening is

necessary on certain highway curves because long vehicles occupy greater width and

the rear wheels generally track inside the front wheels. The added width of pavement

necessary can be computed by geometry for any combination of curvature and

wheelbase. Generally, widening is not required on modern highways with 12-foot lanes

and high type alinement, but for some combinations of speed, curvature, and width, it

may be necessary to widen these highways also. The amount of widening required on

horizontal curves on roads is shown in table 3-1.

3.5.4.2 THIS IS THE WIDENING NORMALLY required for off-tracking and may not

provide clearance where sight is restricted. The additional width should be added to

the inside of the curve, starting with zero at the TS (tangent-spiral), attain the

maximum at the SC (spiral-curve), and diminishing from the maximum at the CS

(curve spiral) to zero at the ST (spiral-tangent) as shown in figure 3-3. Increased sight

distance may be provided by additional widening or by removal of sight obstructions.

The latter is normally recommended because it is generally more economical. Figure

3-4 shows the relation between sight distance along the center line of the inside lane

on horizontal curves and the distance to sight obstructions located inside these curves.

The clear sight distance along the center line of the inside lane on horizontal curves

should equal the minimum stopping sight distance shown in table 1-1 for the design

speed.

3.6 VERTICAL ALINEMENT.

3.6.1 GRADE. Design of vertical alinement involves the establishment of longitudinal

grade or slope for roads, streets, and highways. The key considerations for

determining grades are speed reduction for maximum grade and drainage for

minimum grade.





3.6.1.1 DETERMINING MAXIMUM GRADE. The maximum allowable grade is

dependent on the length the grade is sustained. The critical length of grade is the

distance a design vehicle with a 40,000 pound gross weight and a 100-hp engine can

travel up a designated grade before speed is reduced below an acceptable value

(usually 30 mph). Critical lengths for grades are shown in tables 1-1 and 1-2. If a grade

exceeds critical length, highway capacity is reduced unless a climbing lane is added

for heavy vehicles.

3.6.1.2 DETERMINING MINIMUM GRADE. Tables 1-1 and 1-2 g1ve minimum grades

which are adequate for proper drainage.

3.6.2 CURVES. Generally, vertical curves should be provided at all points on roads or

streets where there is a change in longitudinal grade. The major control for safe

vehicle operation on vertical curves is sight distance, and the sight distance should be

as long as possible or economically feasible. Minimum sight distance required for

safety must be provided in all cases. Vertical curves may be any one of the types of

simple parabolic curves shown in figure 3-5. There are three length categories for

vertical curves: maximum, length required for safety, and minimum. All vertical curves

should be as long as economically feasible. The minimum length of vertical curves is

also shown in tables 1-1 and 1-2.

3.7 CROSS SECTION. Figure~ 3-6 and 3-7 illustrate typical combinations of cross-

section elements for which geometric design criteria are outlined in tables 1-1 and 1-2.

Other combinations of these cross-section elements are illustrated in AASHTO GU-2.

3.7.1 ROADS.

3.7.1.1 NORMAL-CROWN SECTION. The typical road-type, normal crown cross

section shown in figure 3-6 comprises the so-called "streamlined" cross section.

Shoulder edges, channel bottoms, and the intersection of side slopes with original





ground are rounded for simplification of maintenance and appearance. On roads in

open areas rounding of shoulder edges will be restricted to a strip 3 to 4 feet wide at

the intersection of slopes steeper than 2-1/2:1, and only slight rounding will be used at

intersections of slopes flatter than 2-1/2:1.

Table 3-1

Calculated and Design Values for Pavement Widening on Roads and Streets within a

Typical Installation 2-Lane Pavements, One-Way or Two-Way





FIGURE 3-3

METHOD OF LAYOUT OF WIDENING ANDSUPER-ELEVATION OF SPIRAL LANES





FIGURE 3-4

STOPPING SIGHT DISTANCE ON HORIZONTAL CURVES, OPEN ROAD

CONDITIONS





FIGURE 3-5

TYPES OF VERTICAL CURVES





3.7.1.2 SUPERELEVATED SECTION. Figure 3~6 shows the preferable superelevated

cross sections for roads at Army installations. The low side of this cross section is

similar to a normal-crown section except that the shoulder slope on the low side of the

section is the same as the pavement superelevation, except where normal slope is

greater. On the high side of a superelevated section the algebraic difference in cross

slopes at the pavement edge should not exceed about 0.07. The vertical curve should

be at least 4 feet long, and at least the inside 2 feet of the shoulder should be held on

the superelevated slope.

3.7.2 STREETS. Typical street-type cross sections with and without parking are shown

in figure 3-7. Geometric design for the various cross-section elements shown are

presented in table 1-2.

3.8 INTERSECTION CRITERIA.

3.8.1 GENERAL. Practically all highways within many installations will intersect at

grade, and normally the designer will need to consider only plain unsignalized or

signalized intersections. Intersections are normally closely spaced at regular intervals

along streets in built-up areas, and the capacity of these streets will in most cases be

controlled by intersection capacity.

3.8.2 DESIGN CRITERIA. Geometric design criteria for intersections are presented in

AASHTO GD-3, GU-2, SR-2 and the TRB Highway Capacity Manual.





FIGURE 3-6

TYPICAL ROAD-TYPE CROSS SECTIONS





FIGURE 3-7

TYPICAL STREET-TYPE CROSS SECTIONS





3.8.3 REPRESENTATIVE INSTALLATION AREAS EQUIVALENT TO DESIGN

CRITERIA AREAS. Variations in average intersection capacities on one-way and two-

way streets subject to fixed time signal control are shown for general types of areas

within cities in the TRB Highway Capacity Manual. The curves used at a particular

location on typical installations should be selected on the basis of similarity with the

type of area indicated in the TRB Highway Capacity Manual. The following tabulation

indicates areas in which the intersection curves should normally be used.

3.9 CAPACITY OF INTERSECTIONS. The capacity (DHV) shown in tables 1-1 and 1-

2 is for free-flowing highways without intersections at grade or with few crossroads and

minor traffic. These highways have no traffic control signals at intersections (plain

unsignalized intersections), and capacity is affected very little and uninterrupted flow is

assumed. The AASHTO procedure is suggested as a guide in design of intersections.





This tabulation may serve as a general guide for design. of at-grade intersections in

the following manner. If the DHV of traffic at a given intersection 'is approximately

equal to or less than that shown in the tabulation, capacity of the through highway is

based on the DHV shown in tables 1-1 and 1-2, and no intersection capacity analysis

is required. If the DHV of traffic is greater than that shown in this tabulation, the

intersection should be designed as if it were under signal control. The geometric layout

should be made in conjunction with an intersection capacity analysis, as in the

Highway Capacity Manual. The volumes shown in this tabulation have no relation to

warrants for signalization, nor are they indicative of whether or not signalization should

be used. Warrants for traffic control signals are given in ANSI D6.1.

3.10 INTERSECTION CURVES.

3.10.1 MINIMUM EDGE OF PAVEMENT DESIGN. Where it is necessary to provide

minimum space for turning vehicles at unsignalized at-grade intersections, the

AASHTO design criteria presented in GU-2 and SR-2 should be used. The minimum

radius for edge of pavement design on street intersections is 30 feet, which is required

for passenger (P) cars on 90-degree turns. A larger radius should be used if any truck

traffic is expected or turning speeds greater than 10 mph are anticipated. The

minimum radius on road intersections is 50 feet.





3.10.2 MINIMUM CURB RADII. Minimum curb radii are normally used at plain

unsignalized intersections to reduce intersection area and minimize conflict between

pedestrians and vehicles. The curb design should fit the minimum turning path of the

critical design vehicle expected in the traffic. Generally, the minimum curb radii to be

used on intersection curves may be determined on the basis of the following

information.

3.10.2.1 CURB RADII of 15 to 25 feet are adequate for P design vehicles and should

be used on Classes D and E cross streets where practically no single unit (SU) truck,

WB40, WB50, and WB60 (semitrailer combination trucks) design vehicles are

expected or at major intersections where parking is permitted on both intersecting

streets. Radii of 25 feet should be provided on all new construction and on

reconstruction where space is available.

3.10.2.2 CURB RADII of 30 feet or more should be provided at all major highway

intersections to accommodate an occasional truck in the traffic. (See table 2-1 for

minimum turning radius.)

3.10.2.3 RADII OF 40 FEET OR MORE, preferably three-centered compound curves,

to fit the path of the critical design vehicle expected in the traffic, should be provided

where SU, WB40, WB50, and WB60 design vehicles turn repeatedly. (See table 2-1

for minimum turning radius.)

3.11 MISCELLANEOUS.

3.11.1 SIGNING. Signs should conform to ANSI 06.1 standards.

3.11.2 PAVEMENT MARKINGS. Marking should be provided on paved surfaces as a

safety measure and to increase orderly traffic flow. Markings should conform to local

highway practice criteria. Standard requirements are provided in ANSI 06.1 on uniform

traffic control devices.



an introduction to geometric design of roads and streets

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